1. Field of the Invention
The present invention relates to a method of testing an A/D converter circuit, and the A/D converter circuit.
2. Description of Related Art
As known, for example, from “Basic Knowledge of Computer Terminology” by CQ Publishing Co. Ltd, online searched on Apr. 11, 2005 at <URL:http://www.cqpub.co.jp/try/kijidb/yougo/ju.htm>, the A/D converter circuit is mainly classified into “double integral type” as shown in FIG. 5, “sequential comparison type” as shown in FIG. 6A, and “parallel type” as shown in FIG. 6B.
As shown in FIG. 5, the double integral type A/D converter circuit 101 includes an integration circuit 110 constituted mainly by an operational amplifier. A capacitor C connected between the inverting input terminal and the output terminal of the operational amplifier is charged by an input voltage Vin to be A/D converted for a predetermined time period, and then it is discharged at a certain discharge speed by switch over from this input voltage Vin to a reference voltage Vref. During this discharge, the output voltage of the integration circuit 110 is compared with a predetermined threshold voltage (0V, for example) in a comparator 112 to detect a timing at which the output voltage of the integration circuit 110 exceeds the threshold voltage.
A switch control circuit 114 controls the counting operation of a counter 116 on the basis of the output of the comparator 112 and the timing of the switch over between the input voltage Vin and the reference voltage Vref which are applied to the integration circuit 110 alternately. An A/D converted signal of the input voltage Vin can be obtained from a count value of the counter 116 representing a length of a charging period during which the input voltage Vin is applied to the integration circuit 110, and a count value of the counter 116 representing a discharging period between the moment at which the reference voltage Vref is applied to the integration circuit 110 and the moment at which the value of the output voltage of the comparator 112 changes.
In short, since the charged voltage of the capacitor C is an average value of the input voltage Vin during the charging period, and the length of the discharging period is proportional to the charged voltage, A/D converted signal of the input voltage Vin can be determined from these count values of the counter 116. In this double integration type A/D converter 101, the length of the sampling period (the time period necessary to perform a single A/D converting process) has to be set larger than the sum of a length of a discharging period when the input voltage Vin takes its maximum and a certain length of the charging period.
As shown in FIG. 6A, the sequential comparison type A/D converter circuit 102 includes a resistor ladder 120 having resistors ladder-connected with one another, a switching section 122 switching the connection state of the resistor ladder 120, and a comparator 124 comparing an input voltage Vin with a voltage which the resistor ladder 120 serving as a voltage divider circuit generates as a comparison voltage Vref by dividing down a constant voltage (5V in this embodiment).
The sequential comparison type A/D converter circuit 102 performs a first-time comparison in a state where the switching section 122 sets the connection state of the resistor ladder 120 such that a voltage equal to a half of the full scale voltage is generated as a comparison voltage Vref(1). If the input voltage Vin is larger than the comparison voltage Vref(1), the sequential comparison type A/D converter circuit 102 performs a second-time comparison in a state where the switching section 122 sets the connection state of the resistor ladder 120 such that a quarter of the full scale voltage becomes a new comparison voltage Vref(2). On the other hand, if the input voltage Vin is smaller than the comparison voltage Vref(1), the sequential comparison type A/D converter circuit 102 performs a second-time comparison in a state where the switching section 122 sets the connection state of the resistor ladder 120 such that the sum of the current comparison voltage Vref(1) and a quarter of the full scale voltage becomes a new comparison voltage Vref(2).
Thereafter, if the input voltage Vin is larger than a comparison voltage Vref(k) in a k-th time comparison, the sequential comparison type A/D converter circuit 102 performs a (k+1)-th time comparison in a state where the switching section 122 sets the connection state of the resistor ladder 120 such that the sum of a previous comparison voltage Vref (k−1) and a ½k+1 of the full scale voltage becomes a new comparison voltage Vref (k+1). On the other hand, if the input voltage Vin is smaller than the comparison voltage Vref (k) in the k-th time comparison, the sequential comparison type A/D converter circuit 102 performs a (k+1)-th time comparison in a state where the switching section 122 sets the connection state of the resistor ladder 120 such that the sum of the current comparison voltage Vref (k) and a ½k+1 of the full scale voltage becomes a new comparison voltage Vref (k+1).
An A/D converted signal of the input voltage Vin is determined from the connection state of the ladder resistor 120 (opening and closing states of switches of the switching section 122) in a final comparison. Accordingly, the sequential comparison type A/D converter circuit 102 has to repeat the comparison operation by the number of times equal to the number of bits constituting the A/D converted signal. In this sequential comparison type A/D converter circuit 102, a comparison time for one comparison operation is equal to the sum of a time needed to set the switching section 122 and a time needed for the output of the comparator 124 to stabilize after the comparison voltage applied to the comparator 124 is changed. Accordingly, the length of the sampling period (the time period necessary to perform a single A/D converting process) has to be set larger than the comparison time multiplied by the number of the comparison operations performed during the single A/D converting process.
As shown in FIG. 6B, the parallel type A/D converter circuit 103 includes a voltage dividing section 130 generating comparison voltages by equally dividing the full scale voltage into n=2m parts, and a comparison section 132 having n comparators CM1 to CMn for comparing the input voltage Vin with the comparison voltages individually when it is necessary to A/D convert the input voltage Vin into m-bit digital signal.
An A/D converted signal of the input voltage Vin is determined from the output states (high or low) of the comparators CM1 to CMn. Accordingly, the length of the sampling period (the time period needed to perform a single A/D converting process) in the parallel type A/D converter circuit 103 can be set as small as the time needed for the outputs of the comparators CM1 to CMn to stabilize after the comparison voltage applied to these comparators is changed. Hence, the parallel type A/D converter 103 is capable of performing the A/D conversion at high speed.
As clear from the above explanation, each of the double integral type A/D converter circuit 101, the sequential comparison type A/D converter circuit 102, and the parallel type A/D converter circuit 103 has at least one comparator applied with the input voltage Vin at one input terminal thereof. During a test on such A/D converters, it is necessary to confirm that each comparison circuit operates normally for the entire range of the input voltage Vin.
More specifically, when the A/D converter circuit is configured to output an m-bit digital signal as the A/D converted input voltage, it is necessary to repeat a procedure where a voltage applied to the A/D converter circuit as the input voltage Vin is changed stepwise by an amount equal to ½m of the full scale of the input voltage Vin, and the m-bit digital signal outputted from the A/D converter circuit is checked as to whether it matches the voltage applied to the A/D converter circuit each time the voltage applied to the A/D converter circuit is changed.
Accordingly, the number of voltage steps to be checked increases exponentially with the increase of the resolution of the A/D converter circuit, that is, with the increase of the number m (the number of the bits forming the A/D converted signal). Incidentally, to test the A/D converter circuits 101 to 103, it is necessary to change the voltage applied to them stepwise at a resolution higher than at least the resolutions of the A/D converters 101 to 103. Accordingly, to test the A/D converters 101 to 103, an expensive evaluation device having a quite high accuracy has been needed.
In addition, since the sampling time has to be set long in the dual integration type A/D converter circuit 101 and the sequential comparison type A/D converter circuit 102, a time needed to perform the test becomes extremely long when they have high resolution.
The parallel type A/D converter 103, which can operate at high speed, also has a problem in that, when the number of bits forming the A/D converted signal is increased, for example, increased by one, the number of the comparators included therein has to be increased twice, and accordingly the entire circuit scale thereof is almost doubled. This make it difficult for the parallel type A/D converter to have high resolution.